Marketable Securities - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Securities [Line Items]
Net unrealized gain (loss) on securities	$ (41,900,000)	$ 14,700,000	$ (22,600,000)
Equity Securities
Gain (Loss) on Securities [Line Items]
Other-than-temporary Impairment charges	$ 0	$ 0	$ 9,800,000